Propane which is an example of a lower paraffin is used in the field of electronic materials such as semiconductors, and the application requires propane to have a purity as high as possible.
A source gas composed mainly of propane used as a source material for purification contains impurities such as propylene. As methods for purifying a propane gas from the source gas, for example, a method using distillation, a method using membrane separation, a method using adsorptive separation, and the like are known.
The method using distillation requires a large number of theoretical stages for separation because propane and propylene have close boiling points (a difference in boiling point of 4.9° C.). Accordingly, large scale equipment and setting of precise distillation conditions are required, which is a significant barrier to industrial practical application of the method for purifying highly pure propane using distillation.
The method using membrane separation has a low single-pass purification efficiency, and therefore by allowing a source material to pass through a separation membrane a plurality of times for obtaining highly pure propane. However, as the purity of the propane obtained is increased, the recovery rate is reduced to a very low level (see, for example, Patent Document 1).
As adsorptive separation, a method using a silica gel to which silver nitrate has been coated as an adsorbent (see Non-Patent Document 1 below), a method using a silica gel to which silver nitrate has been coated and AlPO-14 as an adsorbent (see Non-Patent Document 2 below), a method using a silica gel to which silver nitrate has been coated and zeolite 4A as an adsorbent (see Non-Patent Document 3 below), and the like are known. However, it is difficult to obtain highly pure propane at a high recovery rate with the methods using adsorbents. This is presumably because, for example, propane is adsorbed on a portion where silver nitrate is not coated, and thus selective adsorption of propylene is not satisfactory.
As described above, a large number of methods have been considered to obtain highly pure propane from a source gas, but there is a trade-off relationship between the recovery rate of propane from a source gas and the purity of propane obtained.
Patent Document 1:
    JP 2006-508176 ANon-Patent Document 1:    New sorbents for olefin/paraffin separations by adsorption via π-complexation: synthesis and effects of substrates, Joel Padin, Ralph T. Yang, Chemical Engineering Science 55 (2000) 2607-2616Non-Patent Document 2:    Propane/propylene separation by pressure swing adsorption: sorbent comparison and multiplicity of cyclic steady states, Salil U. Rege, Ralph T. Yang, Chemical Engineering Science 57 (2002) 1139-1149Non-Patent Document 3:    Molecular sieve sorbents for kinetic separation of propane/propylene, Joel Padin, Salil U. Rege, Ralph T. Yang, Linda S. Cheng, Chemical Engineering Science 55 (2000) 4525-4535